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United States Patent |
6,189,993
|
Mantell
|
February 20, 2001
|
Ink jet printer having multiple level grayscale printing
Abstract
A liquid ink printing system for printing images having different
grayscales through control of a printhead carriage velocity and the number
of drops deposited per pixel location. The liquid ink printing system
prints from print data an image on a recording medium including pixel
locations being deposited with liquid ink drops. The printing system
includes a printhead, including a plurality of drop ejectors, for
depositing the liquid ink drops, a scanning carriage, coupled to the
printhead, for moving the printhead across the recording medium, and a
print driver, operatively coupled to the scanning carriage and to the
printhead, for controlling the scanning carriage to move at a plurality of
velocities, each of the plurality of velocities being associated with one
of a plurality of grayscales. Each of the plurality of grayscales is
characterized by a maximum number of ink drops deposited at one of the
pixel locations. In a draft mode, for instance, one drops per pixel
location is deposited and in a high quality mode, four drops per pixel
location are deposited. For each of the grayscale modes, the scanning
carriage moves at a different velocity.
Inventors:
|
Mantell; David A. (Rochester, NY)
|
Assignee:
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Xerox Corporation (Stamford, CT)
|
Appl. No.:
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829033 |
Filed:
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March 31, 1997 |
Current U.S. Class: |
347/15 |
Intern'l Class: |
B41J 002/205 |
Field of Search: |
347/15,37,43,50,12,41,40,14,23
|
References Cited
U.S. Patent Documents
4631548 | Dec., 1986 | Milbrandt | 347/15.
|
4965593 | Oct., 1990 | Hickman | 346/140.
|
5216445 | Jun., 1993 | Hirsaswa et al. | 347/15.
|
5245359 | Sep., 1993 | Ito et al. | 347/37.
|
5412410 | May., 1995 | Rezanka | 347/15.
|
Foreign Patent Documents |
623473 A2 | Sep., 1994 | EP.
| |
Primary Examiner: Barlow; John
Assistant Examiner: Stewart, Jr.; Charles W.
Attorney, Agent or Firm: Arthur; David J.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
Cross-reference is made U.S. patent application entitled "Liquid Ink
Printing With Small Drop Overprinting", Ser. No. 08/673,517, to David
Mantell, and to U.S. patent application entitled "Checkerboard Printing
for Multiple Drops per Pixel Ink Jet Printing", Ser. No. 08/781,365,, to
David Mantell, both assigned to Xerox Corporation herein incorporated by
reference.
Claims
What is claimed is:
1. A liquid ink printing system for multiple level grayscale printing, from
print data, an image on a recording medium, the recording medium moving in
a first direction along a path, and including pixel locations thereon for
receipt of liquid ink drops, comprising:
a printhead, located adjacent the path, including a plurality of drop
ejectors, for selectively depositing one or more liquid ink drops per
pixel location on the recording medium;
an electromover, for moving the recording medium along the path;
a user interface having a plurality of print quality modes and a plurality
of types of recording medium presented for selection by a user prior to
printing;
a print driver, operatively coupled to said electromover, user interface,
and to said printhead, the print driver having means for selecting one of
a plurality of relative velocities between the recording medium and said
printhead in response to a selection of a print quality mode and a type of
recording medium from the user interface by a user, said plurality of
relative velocities being previously stored in the print driver with each
of said plurality of relative velocities being associated with one of a
plurality of grayscales, whereby the print driver controls the relative
velocity between the printhead and recording medium at the relative
velocity and associated grayscale selected by said selecting means.
2. The printing system of claim 1, comprising a scanning carriage, coupled
to said printhead, for moving said printhead across the recording medium
in a second direction substantially perpendicular to the first direction.
3. The printing system of claim 2, wherein the selecting means of said
print driver includes a velocity signal selector, for generating a
plurality of velocity signals, each of said plurality of velocity signals
being generated to move said scanning carriage at one of said plurality of
relative velocities.
4. The printing system of claim 3, wherein the selecting means of said
print driver further includes a grayscale selector, for generating a
plurality of grayscale signals, each of said plurality of grayscales being
generated to print the image at one of said plurality of grayscales.
5. The printing system of claim 4, wherein each of said grayscale signals
is for specifying a maximum number of liquid ink drops per pixel for
deposition in each of said plurality of grayscales.
6. The printing system of claim 5, wherein said velocity signal selector
generates a first velocity signal associated with the deposition of N
drops per pixel and a second velocity signal associated with the
deposition of Y drops per pixel.
7. The printing system of claim 6, wherein said first velocity signal
causes said scanning carriage to move at a first velocity; wherein said
second velocity signal causes said scanning carriage to move at a second
velocity; and wherein the first velocity is greater than the second
velocity, and the N drops per pixel is less than the Y drops per pixel.
8. The printing system of claim 1, wherein each of said plurality of
grayscales corresponds to a maximum number of ink drops being deposited at
the pixel locations.
9. The printing system of claim 8, wherein said print driver generates a
frequency control signal for controlling the drop ejection frequency as a
function of one of said plurality of relative velocities.
10. A liquid ink printing system for multiple level grayscale printing of
an image on a recording medium in response to print data received, the
recording medium being movable along a path in a first direction and
including pixel locations thereon, said printing system comprising:
a user interface having a plurality of print quality modes, each of which
represents a level of grayscale printing, and a plurality of types of
recording medium, the user interface being used by a user to select one of
the print quality modes and one of the types of recording medium prior to
printing by the printing system;
a printhead having a plurality of drop ejectors for selectively depositing
one or more liquid ink drops per pixel location on the recording medium,
the printhead being located adjacent the path along which the recording
medium is moved;
an electromover for moving the recording medium along the path; and
a print driver being operatively coupled to the printhead, user interface,
and electromover, the print driver having a velocity selector with a first
storage means containing a plurality of predetermined relative velocities
between the printhead and recording medium stored therein and a grayscale
selector with a second storage means containing a plurality of
predetermined number of drops per pixel location stored therein, the
velocity selector and grayscale selector each respectively analyzing a
print quality mode and a type of recording medium selected by the user,
the velocity selector selecting a one of the predetermined relative
velocities in the storage means and generating a velocity signal
indicative thereof, the grayscale selector selecting a one of the
predetermined number of drops per pixel location stored in said second
storage means and generating a grayscale signal representative thereof,
the print driver controlling the relative velocity between the printhead
and recording medium and the number of drops per pixel location during
printing of the image on the recording medium by the printhead in
accordance with both the velocity signal and the grayscale signal.
11. The printing system as claimed in claim 10, wherein the printhead is a
stationary, pagewidth printhead and the recording medium is moved past the
pagewidth printhead in said first direction at a constant velocity
selected by the velocity selector of the print driver.
12. The printing system as claimed in claim 10, wherein the printhead is
mounted on a reciprocating carriage and the recording medium is held
stationary while the printhead is moved in a second direction which is
perpendicular to the first direction, the moving printhead printing a
swath of image on the recording medium at the velocity selected by the
print driver, the recording medium being advanced in said first direction
after the swath is printed by the distance of a height of a printed swath,
during which advancing of the recording medium the printhead does not
print.
13. The printing system as claimed in claim 10, wherein the first and
second storage means are first and second lookup tables, respectively; and
wherein each grayscale signal specifies a maximum number of liquid ink
drops per pixel location to be deposited.
14. The printing system as claimed in claim 13, wherein said relative
velocity is slowed by a factor of N where N is the number of drops per
pixel location.
15. A method for multiple level grayscale printing of an image on a
recording medium by a liquid ink printing system in response to print data
received by said printing system, the recording medium being movable along
a path in a first direction and including pixel locations thereon, the
method for printing comprising the steps of:
(a) providing a user interface having a plurality of print quality modes,
each of which represents a level of grayscale printing, and a plurality of
types of recording medium;
(b) providing a printhead having a plurality of drop ejectors adjacent the
path along which the recording medium is moved, each ejector being adapted
to deposit one or more liquid ink drops per pixel location on the
recording medium;
(c) moving the recording medium along the path by an electromover;
(d) operatively coupling a print driver to the printhead, user interface,
and electromover, said print driver having a velocity selector with a
first storage means and a grayscale selector with a second storage means;
(e) entering a plurality of predetermined relative velocities between the
printhead and recording medium in said first storage means and entering a
plurality of predetermined number of drops per pixel location in said
second storage means;
(f) selecting a one of the print quality modes and a one of the types of
recording medium from the user interface prior to printing by the printing
system;
(g) analyzing a print quality mode and a type of recording medium selected
from the user interface by both the velocity selector and the grayscale
selector in order to enable a selection from the respective first and
second storage means by said velocity selector and grayscale selector;
(h) selecting a one of the plurality of predetermined relative velocities
stored in the first storage means by the velocity selector and selecting a
one of the plurality of predetermined number of drops per pixel location
stored in the second storage means by the grayscale selector;
(i) generating a velocity signal indicative of said selected relative
velocities by the velocity selector and generating a grayscale signal
indicative of the selected number of drops per pixel location; and
(j) controlling the relative velocity between the printhead and recording
medium and the number of drops per pixel location during printing of the
image on the recording medium by the printhead in response to the velocity
signal and the grayscale signal.
16. The method as claimed in claim 15, wherein the first and second storage
means are first and second lookup tables, respectively; and wherein each
grayscale signal specifies a maximum number of liquid ink drops per pixel
location to be deposited.
17. The method as claimed in claim 16, wherein said relative velocity is
slowed by a factor of N where N is the number of drops per pixel location.
Description
FIELD OF THE INVENTION
This invention relates generally to a method and apparatus for liquid ink
printing and more particularly to a printing system for printing images
having different grayscales through control of printhead carriage velocity
and the number of drops deposited per pixel.
BACKGROUND OF THE INVENTION
Liquid ink printers of the type frequently referred to as continuous stream
or as drop-on-demand, such as piezoelectric, acoustic, phase change
wax-based, or thermal, have at least one printhead from which droplets of
liquid ink are directed towards a recording medium. Within the printhead,
the ink is contained in a plurality of ink conduits or channels. Power
pulses cause the droplets of ink to be expelled as required from orifices
or nozzles at the ends of the channels.
In a thermal ink-jet printer, the power pulse is usually produced by a
heater transducer or a resistor, typically associated with one of the
channels. Each resistor is individually addressable to heat and vaporize
ink in the channels. As voltage is applied across a selected resistor, a
vapor bubble grows in the associated channel and initially bulges toward
the channel orifice followed by collapse of the bubble. The ink within the
channel then retracts and separates from the bulging ink thereby forming a
droplet moving in a direction away from the channel orifice and towards
the recording medium whereupon hitting the recording medium a dot or spot
of ink is deposited. The channel is then refilled by capillary action,
which, in turn, draws ink from a supply container of liquid ink.
The ink jet printhead may be incorporated into either a carriage type
printer, a partial width array type printer, or a page-width type printer.
The carriage type printer typically has a relatively small printhead
containing the ink channels and nozzles. The printhead can be sealingly
attached to a disposable ink supply cartridge and the combined printhead
and cartridge assembly is attached to a carriage which is reciprocated, at
a constant speed, to print one swath of information (equal to the length
of a column of nozzles), at a time, on a stationary recording medium, such
as paper, fabric, or a transparency. After the swath is printed, the paper
is stepped a distance equal to the height of the printed swath or a
portion thereof, so that the next printed swath is contiguous or
overlapping therewith. This procedure is repeated until the entire page is
printed. In contrast, the page width printer includes a stationary
printhead having a length sufficient to print across the width or length
of the recording medium at a time. The recording medium is continually
moved past the page width printhead in a direction substantially normal to
the printhead length and at a constant or varying speed during the
printing process. A page width ink-jet printer is described, for instance,
in U.S. Pat. No. 5,192,959, herein incorporated by reference.
Printers typically print information received from an image output device
such as a personal computer. Typically, this received information is in
the form of a raster scan image such as a full page bitmap or in the form
of an image written in a page description language or a combination
thereof. The raster scan image includes a series of scan lines consisting
of bits representing pixel information in which each scan line contains
information sufficient to print a single line of information across a page
in a linear fashion. Printers can print bitmap information as received or
can print an image written in the page description language once converted
to a bitmap consisting of pixel information.
Information printed by a printer can be printed having the grayscale of the
received information. The printer can also modify the received bitmap and
print the information at a grayscale different than the one received. In
either event, it is typical that grayscale printing is performed either by
controlling the number of drops per pixel, by controlling the size of the
white space between pixels, or both. One known method of improving image
quality through grayscale control is to print pixels on a higher
resolution grid in both the scan direction and the paper advance
direction. Such methods require multiple passes of the printhead so that
more drops are placed within a given region of the print medium. These
methods also require small and very accurate paper advance steps for
accurate placement of the drops on a high resolution grid. Because such
methods require multiple passes of the printhead, print speed may be
undesirably reduced. In addition, accurate paper advance steps may
increase the cost of the printing system as such systems typically require
advanced electronic controllers and expensive encoders which can be cost
prohibitive.
Various methods and apparatus for printing images with scanning carriage
type liquid ink printers have been developed. The following references
describe these and other methods and apparatus for liquid ink printing.
U.S. Pat. No. 4,965,593 to Hickman describes a dot printer wherein the
spacing of ink jet nozzles of a print head are spaced by an amount greater
than the pixel spacing of the printing medium such that adjacent pixels
are not printed until the deposited colorant has time to dry.
European Patent Application Publication No.623 473 to Holstun et al,
describes increased print resolution in the carriage scan axis of an
ink-jet printer. The increased print resolution is achieved by moving the
carriage of an ink-jet cartridge in the carriage scan direction to provide
a first resolution in that direction which is twice the second resolution
in a print media advance direction. Two smaller drops of ink are fired
onto each square pixel in a single pass of the cartridge so as to provide,
for example, a 600 dpi resolution in the carriage scan axis with a 300 dpi
resolution in the media advance direction.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, there is provided a
liquid ink printing system for printing, from print data, an image on a
recording medium, moving in a first direction along a path, including
pixel locations being deposited with liquid ink drops. The printing system
includes a printhead, located adjacent the path, including a plurality of
drop ejectors, for depositing the liquid ink drops, an electromover, for
moving the recording medium along the path, and a print driver,
operatively coupled to the electromover and to the printhead, for
controlling a plurality of relative velocities between the recording
medium and the printhead, each of the plurality of relative velocities
being associated with one of a plurality of grayscales.
Pursuant to another aspect of the present invention, there is provided a
method of printing with a liquid ink printhead including drop ejectors
ejecting ink drops at a drop ejection frequency at a plurality of pixel
locations having a predetermined resolution on a recording medium moving
along a path to form an image including a plurality of dots. The steps
include examining an image quality selection, determining, as a function
of the examined image quality selection, a relative velocity between the
recording medium and the printhead for printing the image, and printing
the image with the printhead ejecting ink drops on the recording medium
with the relative velocity between the recording medium and the printhead
being determined in said determining step.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a partial schematic perspective view of a printing system
incorporating the present invention.
FIG. 2 is a schematic perspective view of an ink jet print cartridge having
an ink jet printhead with ink ejecting nozzles and associated heaters
incorporating the present invention.
FIG. 3 illustrates the locations of ink drops deposited by the printhead on
a plurality of recording mediums according to the present invention.
FIG. 4 illustrates a schematic block diagram of a printing system including
a print driver according to the present invention.
FIG. 5 illustrates an embodiment of a user interface for use in the present
invention.
While the present invention will be described in connection with a
preferred embodiment thereof, it will be understood that it is not
intended to limit the invention to that embodiment. On the contrary, it is
intended to cover all alternatives, modifications, and equivalents as may
be included within the spirit and scope of the invention as defined by the
appended claims.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 illustrates a partial schematic perspective view of a printing
system including a personal computer 8, generating print data, coupled to
one type of liquid ink printer, an ink jet printer 10, having an ink jet
printhead cartridge 12 mounted on a carriage 14 supported by carriage
rails 16. The printhead cartridge 12 includes a housing 18 containing ink
for supply to a thermal ink jet printhead 20 which selectively expels
droplets of ink under control of electrical signals received from a
controller 21 of the printer 10 through an electrical cable 22. The
signals generated by the controller 21 are generated in response to the
print data generated by the personal computer 8 as is understood by one
skilled in the art. Other image input devices are also possible, of
course, such as a scanner, other computer image generators, and image
storage devices. Such image data may include color information or
monochrome information for printing by a color capable liquid ink printer.
The printhead 20 contains a plurality of drop ejectors, including ink
conduits or channels (not shown) which carry ink from the housing 18 to
respective ink ejectors, which eject ink through orifices or nozzles (also
not shown). When printing, the carriage 14 reciprocates or scans back and
forth along the carriage rails 16 in the directions of the arrow 24 at a
constant speed or velocity. As the printhead cartridge 12 reciprocates
back and forth across a recording medium 26, such as a sheet of paper or
transparency, droplets of ink are expelled from selected ones of the
printhead nozzles towards the sheet of paper 26. The ink ejecting orifices
or nozzles are typically arranged in a linear array substantially
perpendicular to the scanning direction 24. If printing in color, such a
linear array can be segmented such that segments of the array deposit
different colors of ink to complete a color image. Such a printer would
necessitate that the housing 18 include separate portions or containers
each including a different type or color of ink. During each pass of the
carriage 14, the recording medium 26 is held in a stationary position. At
the end of each pass, however, the recording medium is advanced or stepped
in a paper advance direction 28 by a stepping mechanism or electromover
under control of the printer controller 21. For a more detailed
explanation of the printhead and printing thereby, refer to U.S. Pat. No.
4,571,599, U.S. Pat. No. Reissue 32,572, and U.S. Pat. No. 5,534,895 each
of which are incorporated herein by reference.
It is well known and commonplace to program and execute imaging, printing,
document, and/or paper handling control functions and logic with software
instructions for conventional or general purpose microprocessors, such as
the controller 21. This is taught by various prior patents and commercial
products. Such programming or software may of course vary depending on the
particular functions, software type, and microprocessor or other computer
system utilized, but will be available to, or readily programmable without
undue experimentation from, functional descriptions, such as those
provided herein, or prior knowledge of functions which are conventional,
together with general knowledge in the software and computer arts. That
can include object oriented software development environments, such as
C++. Alternatively, the disclosed system or method may be implemented
partially or fully in hardware, using standard logic circuits or a single
chip using VLSI designs.
The carriage 14 is moved back and forth in the scanning directions 24 by a
belt 38 attached thereto. The belt 38 is moved by a first rotatable pulley
40 and a second rotatable pulley 42. The first rotatable pulley 40 is, in
turn, driven by a reversible motor 44 under control of the controller 21
of the ink jet printer. To enable the present invention, the motor 44 is a
variable speed motor which can be operated at different speeds to move the
carriage 14 at different velocities. In addition to the toothed
belt/pulley system for causing the carriage to move, it is also possible
to control the motion of the carriage by using a cable/capstan, lead screw
or other mechanisms as known by those skilled in the art.
To control the movement and/or position of the carriage 14 along the
carriage rails 16, the printer includes an encoder having an encoder strip
46 which includes a series of fiducial marks in a pattern 48. The pattern
48 is sensed by a sensor 50, such as a photodiode/light source attached to
the printhead carriage 14. The sensor 50 includes a cable 52 which
transmits electrical signals representing the sensed fiducial marks of the
pattern 48 to the printer controller to thereby measure actual printhead
position. Highly accurate encoders are desirable as knowledge of actual
printhead position improve the operation of the present invention. Other
known encoders, such as rotary encoders are also possible.
FIG. 2 illustrates the printhead 20 of the present invention which includes
a plurality of nozzles 60 which are spaced a distance S which may be
equivalent to a desired resolution of the image in the paper advance or
non-scan direction. Partial steps in the paper advance direction are also
possible to increase the resolution of the printed image over that of the
nozzle spacing. For instance, the spacing S could be 300 spots per inch in
the paper advance direction.
In determining the amount of ink necessary to be deposited by the nozzles
60 for grayscale printing of the present invention, it is important to
determine how much optical density, and therefore ink volume, is necessary
to provide the same amount of ink coverage in the known 300 by 300
resolution printing. It has been found that it takes approximately the
same amount of ink or less to cover the page by printing successive small
drops as for binary printing. Thus as long as the drop size can be
decreased and the firing frequency increased, grayscale printing becomes a
viable method for improving image quality. Smaller drop volumes also
provide the additional benefit of faster refill time or recovery time for
drop ejectors. The smaller drops also require less energy per drop thereby
enabling more drop ejectors to be energized simultaneously. Reducing the
spot size improves image quality regardless of whether it is reduced from
300 spots per inch to a 600 spot per inch spot size still spaced at 300
spots per inch or to some intermediate spot size. The lightest gray level
and the spacing between gray levels is determined by the spot size.
Consequently, the described invention can provide significant improvements
in grayscale.
Since the printhead 20 includes smaller nozzles, multiple drops are
necessary to fill a desired pixel since the spacing of the nozzles
dictates the size of the pixel grid unless fractional step advances are
used. As used herein, a "pixel" refers to an image signal associated with
a particular position in an image having an optical density between a
minimum and a maximum. Accordingly, pixels are defined by intensity and
position. The pixels may refer to a particular position, region, or
superpixel (collection of pixels) in the output image. The image data
pixels are commonly grouped into separations. Each separation provides a
set of image signals or separation pixels. These might be used to drive a
printer to produce one color separation of the image. In the case of
multi-color printers, these separations, superimposed together, form the
color image. In this context, pixels are described as discrete image
signals which represent optical density of the document image in a given
small area. In this description, the term gray will be used for pixel
values which vary between maximum and minimum, irrespective of the color
of the separation in which the signals are used. In addition, the present
invention is not limited to a color space using cyan, magenta, yellow, and
black but also other color spaces such as color spaces having more than 4
colorants, other common color spaces, such as r, g, b, or color spaces
such as those including luminance, chroma, and hue. Likewise, the present
invention includes the use of any colorants or inks such as cyan, magenta,
yellow, and black but also including but not limited to orange, red,
green, blue, light cyan, light magenta, light yellow, and light black or
gray.
For purposes of illustration, assuming the nozzle spacing of the printhead
20 is 300 spots per inch, it is possible to achieve a number of grayscales
or gray levels by selecting the number of drops per pixel which are to be
deposited. For instance as illustrated in FIG. 3, multiple grayscales are
possible. A recording medium 26A, for instance, illustrates a first level
of grayscale, which might also be considered to be a draft mode, wherein
only a single drop 64 of ink (illustrated before spreading) is deposited
on any one of a plurality of pixel areas 66 (one illustrated). Such a
draft mode might be useful to preview the alignment of image information
on a completed document. For another recording medium 26B, a second level
of grayscale can be achieved by depositing no more than two drops 68 of
ink on any one of the pixel locations 66. In a third level of grayscale, a
recording medium 26C has no more than three drops 70 deposited in any one
of the pixel areas 66. As a final example, no more than four drops 72 of
ink are deposited on any of the pixel locations 66 of a recording medium
26D.
The present invention is also applicable to other printing methods,
including known checkerboard printing and others such as that described in
previously referred to U.S. patent application "Checkerboard Printing for
Multiple Drops per Pixel Ink Jet Printing". Checkerboard printing is
enabled in any of the multiple drop per pixel grayscale resolutions by
depositing ink at non-adjacent pixel locations, in the horizontal and
vertical directions, in a single pass of the printhead, where one or more
pixel locations are left un-printed during a single pass. A second pass or
more of the printhead, after a partial advance of the recording media
completes the pixel locations.
It has been found, that grayscale printing with liquid ink can be greatly
improved by depositing a number of small ink drops unequally or equally
spaced within a pixel space where each drop has a different drop center
but which are clustered near the center of the pixel space, such as
described in U.S. patent application "Liquid Ink Printing With Small Drop
Overprinting", Ser. No. 08/673,517, to David Mantell. Such drops are
deposited in rapid succession within the pixel space such that ink of each
drop mixes and spreads into a larger single spot. Most inks will spread
more in the direction perpendicular to the printhead motion since the
drops are already spread out in the direction of motion. Only inks that
effectively do not spread at all (very slow dry inks) or inks which finish
spreading faster than the drops can be deposited (extremely fast dry ink)
would be excluded. Thus the drops will tend toward the size and shape of a
single drop having the same amount of ink, only slightly elongated in the
printhead motion direction.
The described ink spreading phenomenon occurs because at the high drop
frequencies of an ink jet printer, typically greater than 5 kHz, the drops
arrive on the paper faster than they are able to spread. For instance, the
single ink drop 64 (illustrated as an actual ink drop before spreading)
will have a size slightly larger than the initially deposited drop due to
drop spreading but which will be smaller in size than the size of an ink
drop deposited by a single nozzle depositing a standard drop size in a 300
spot per inch printer. The two ink drops 68 will result in a drop larger
than that achieved by the single drop 64. The ink spreads in the paper
advance direction more so than in the scanning direction and the resulting
drop includes a fairly circular appearance.
Paper type can also be an important factor in drop spreading. For instance,
it has been found that Springhill 6T paper wets more rapidly than Xerox
Recycled paper. Consequently, by the selection of paper and the selection
of the liquid ink, drop spreading can be optimized to achieve desired
printing results. For instance, different quality printing modes, such as
draft mode or high quality mode, can be made by the selection of recording
mediums as well as inks.
The ink jet printer 10 is included in a basic image processing system 80 of
FIG. 4. In the system 80, an electronic representation of a document or
image from the personal computer 8 here illustrated as print data 82, is
transferred to a print driver 84. The print data 82 includes electronic
digital data in some form from an original image or from another source,
in a format related to the physical characteristics of the personal
computer, typically including pixels. While a personal computer is shown,
other known image input terminals, including a scanner, other computer
image generators, or image storage devices are also possible. In the
instance of the personal computer 8, a user interface, such as illustrated
in FIG. 5, is present on the display device of the personal computer 8 for
providing for a variety of user selections for printing documents. The
user interface 86, is likewise connected to the print driver 84, also
known as an image processing unit. The print driver 84 processes the print
data 82 in accordance with items selected by the user on the user
interface 86 as well as other processes to be described. Once the print
data has been processed by the print driver 80, the processed information
is transmitted to the printer 10 here schematically illustrated to include
the printer controller 21 being coupled to the printhead 20, the scanning
carriage 14 and the encoder 46. While manipulation of the print data is
shown to occur in the print driver 84, it can also occur in the image
output terminal or printer 10.
The present invention includes a print driver which can be accessed through
the user interface 86 to set the level of grayscale to be used for a
particular image. In addition, the level of grayscale selected is used to
control the velocity of the carriage 14 as it scans across the page, such
that a range of grayscales or print modes are available to the user
depending on the desired quality of output in the final document. By
controlling the carriage speed relative to the grayscale, print quality
selected, or printing method, such as checkerboarding, a number of
advantages are possible. One is that a wide variety of levels of high
quality grayscale are possible with a single printer. The present
invention includes a printhead carriage capable of scanning at multiple
velocities. Two, given that there are a range of ink jet papers available
to the user, the user can choose the number of drops printed per pixel
that matches the choice of recording medium. In this manner, the printer
can accept new paper products which were not in existence at the time the
printer was produced. Third, ink savings of the economy modes, such as a
draft mode, are greater because of the smaller than normal drop volumes
deposited by the printhead. Fourth, the draft mode prints at a much faster
rate than comparable products that print with larger standard sized drop
volumes.
One important characteristic of the present invention is that the velocity
of the carriage is set by the print driver according to the level of
grayscale which is selected. For instance, the user selects from a first
screen 90 (see FIG. 5) of the user interface 86, one of a plurality of
print quality modes 92, which can include, for instance, a draft mode 94,
a normal mode 96, a high quality mode 98 and a deluxe quality mode 100. In
addition, when one of the print quality modes 92 is selected, one of a
plurality of media types 102 can be selected which include the selection
of plain paper 104, coated paper 106, glossy high resolution paper 108 and
glossy deluxe resolution paper 110. Print quality mode selections as well
as media type selections affect image quality. It is also possible, that
upon selection of one of the print quality modes 92, the print driver 84
might automatically highlight one of the media types 102 as a
recommendation to the user to use that type of media when printing in the
selected print quality mode. Upon selection of one of the print quality
modes and one of the media types, the user would, if satisfied with the
selections, would select the OK selector 112 to begin printing.
Returning to FIG. 4, the print driver 84 receives the information from the
user interface 86 and stores the transmitted media type in a media type
buffer 120 and stores the selected print quality type in a print quality
type buffer 122. A carriage velocity selector 124 then analyzes the media
type as well as the print quality type. Based upon the combination
thereof, one of four or more print carriage velocities stored in, for
instance, a velocity lookup table 126 are selected. A carriage velocity
signal is subsequently transmitted to the printer 10 in accordance with
the selected velocity. It is also possible for the print driver to
generate one of the plurality of relative velocities as a function of the
number of passes of the printhead across a portion of the recording medium
when, for instance, printing in a checkerboarding mode.
Each of the velocities corresponds to one of the print modes previously
described in FIG. 3. Velocity 1 might be the fastest velocity of the print
carriage and would correspond to the print mode shown for the recording
medium 26A. Velocity 2 would be slower than velocity 1 and would be
applied to the printhead carriage during printing of the recording medium
26B. Continuing along the same line, velocity 3 would be used during
printing of the recording medium 26C and velocity 4 would be applied to
printing of the recording medium 26D. In this fashion, the relative
velocity between the printhead and the media are changed depending on the
level of grayscale desired. Consequently, the present invention is also
applicable to pagewidth printers where a pagewidth printhead is held
stationary and the recording medium is moved past the stationary printhead
at different velocities wherein each velocity corresponds to a desired
grayscale.
To enable printing with various carriage speeds as described, it is
necessary to establish the maximum firing frequency of the printhead as
well as to determine the electronic accessing frequency of the
transmission of any print information which is transmitted to the
printhead 20 for causing the drop ejectors to eject ink. If the carriage
speed is such that a one pixel distance is traveled at the maximum firing
frequency, there is no high addressability. For the present invention,
however, the maximum firing frequency is used to determine a maximum
carriage velocity to be used by the printer in, for instance, the draft
mode. The other carriage velocities are then determined based on the
maximum velocity. For instance, the carriage velocity is slowed by a
factor of N depending on which of the print quality modes are selected,
where N is the number of drops being deposited per pixel.
Frequently, the gear ratios of the motors used to control the carriage
velocity do not allow for a continuous variation of the carriage velocity.
Thus, it may be necessary to adjust the drop ejection or firing frequency
in order to match the carriage velocity to the number of drops being
deposited. In order to keep the firing frequency below the maximum
allowable frequency, this usually means that the firing frequency is
lowered to accommodate for a carriage velocity matched to the particular
motor. This can be accomplished by changing the frequency at which the
electronic control circuitry addresses, from one group of drop ejectors to
another, a complete printing cycle from one end of the printhead to the
other, also known as "ripple". Alternatively, this can be done by
inserting a time delay or pause between successive ripples of the
printhead control circuitry. Other conditions might also require a change
in firing frequency, such as the thermal load on the printhead, as is
known in the art of ink jet printing. On the other hand, sometimes the
nominal maximum continuous firing frequency can be exceeded for short
bursts, as might occur in checkerboard printing. Thus a new effective
maximum firing frequency is established for such printing modes. This
allows additional room for adjustment of the firing frequency once all of
the other printing parameters are determined. Those parameters are the
number of drops per pixel deposited in a pass of the printhead across the
recording medium and the available carriage velocities. In other words,
the frequency is adjusted to be equal to or greater than the velocity
times the number of drop per pixel per pass.
As an example, take the case of a printer providing multiple drops per
pixel through higher resolution in the carriage scan direction. The common
method of achieving two pass checkerboarding is to print every other drop
along a horizontal scan line on each pass of the printhead. In this case,
if the carriage velocity is kept constant from the first pass to the
second pass, then the effective firing frequency is reduced by 2 times.
But, for the present invention, it is possible to increase the carriage
speed by 2 times. This maintains the same firing frequency as in the one
pass printing mode and preserves the overall throughput of the printer.
Eventually, for higher multiple pass modes (e.g. 4 pass printing) a limit
is reached for the maximum carriage velocity. Thus the firing frequency in
those modes are not limited by the maximum firing frequency of the
printhead but by the maximum carriage velocity.
The first enabler of multiple carriage velocities is that the printhead 20
is designed to produce smaller drops than a standard printhead printing
one drop per pixel for a given printer resolution. Because the printhead
is designed to produce smaller drops, the natural firing frequency and the
number of drop ejectors that can be filed simultaneously are greater,
thereby in effect, compensating for the larger number of drops printed per
pixel. In this manner, the throughput can be kept high even though more
drops are printed per pixel and the ink loading on the paper can still be
kept roughly constant. The second enabler is that the carriage velocity is
set by the print driver 84 through selections on the user interface 86
such that the grayscale is controlled by the print driver. As an example,
the draft mode would print one drop per pixel, or for instance a minimum
number of drops per pixel N, the normal mode would print two drops per
pixel, the high quality mode would print three drops per pixel and the
deluxe quality mode would print four drops per pixel, or for instance a
maximum number of drops per pixel Y as illustrated in FIG. 3 for the
recording medium 26D. It is also possible that even higher quality print
modes can be achieved using more drops per pixel and special papers which
effectively cause reduced spot sizes thereby enabling an even greater
number of drops to be printed per pixel. In this fashion, not only does
the user indirectly select the velocity of the printhead carriage, but
also indirectly selects the level of grayscale and the number of bits per
pixel. By including multiple levels of grayscale, it is also possible to
reduce the special paper requirements since grayscale is not completely
coupled to the type of paper being used but depends on the number of drops
per pixel being deposited. In fact, the user could determine the darkness
of the black level and how saturated the colors appear by choosing the
level of grayscale which relates, of course, to the number of drops per
pixel.
To enable the selection of the number of drops per pixel, the print driver
84 includes a grayscale selector 128 which analyzes the media type
selected as well as the print quality selected to select one of a
plurality of drops per pixel from a drops per pixel lookup table 130. Each
print quality mode can be characterized by the maximum number of drops per
pixel which can be deposited during the formation of an image. For
instance, when printing in a draft mode no more than one drop per pixel is
printed at any one of the pixel locations. In a hardware implementation of
the present invention, the carriage velocity selector 124 and lookup table
126 as well as the grayscale selector 128 and lookup table 130 could be
embodied in a multiplexor demultiplexor combination for selection of the
proper velocity as well as number of drops per pixel.
After the drops per pixel have been selected, a CMYK device independent
version of the print data 82, which has been generated by an RGB to CMYK
converter 132, is converted to device dependent data necessary for
printing a final image by the printer 10. The conversion is performed in a
rendering converter 134 as is known by those skilled in the art which can
include resolution conversion 136, color correction 138, and halftoning
140. Once the correction has been made the transformed print data is now
transmitted to the printer controller 21 and includes signal information
reflecting the maximum number of drops per pixel to be used when printing
the image. The printer controller 21 then controls the scanning carriage
velocity according to the velocity signal transmitted from the printer
driver and also controls the ejection of ink by the printhead 20 such that
multiple drops or single drops per pixel are emitted from the printhead in
accordance with the selected print quality as well as media types. In
addition, the printer controller 21 can includes a frequency controller
142 if the frequency needs to be adjusted to match the available carriage
speeds which can be accurately produced by a given carriage drive motor.
If so, the velocity signal or a frequency control signal generated by the
print driver 84 and transmitted to the printer 10 can include signal
information for controlling drop ejector frequency. The encoder 46 ensures
accurate positioning of the printhead as the printhead scans across the
recording sheet such that the placement of multiple or single drops with
any single pixel is accurately controlled.
The present invention utilizes higher quality encoders that measure the
actual printhead position, while avoiding the use or necessity of using
higher quality types of encoders tied to the paper advance mechanism. This
efficiently optimizes the use of multiple levels of grayscale within a
single printer, because precision motion in the paper advance direction is
not necessary. For instance, print modes requiring higher addressability
in a paper advance direction such as a 1200.times.1200 mode using a 600
dot per inch printer would require additional cost encoders in the paper
advance direction and would be unlikely to have the same performance
specifications for controlling motion in the carriage direction.
Consequently, the present invention of changing carriage velocity with
respect to print mode as well as controlling the number of drops per pixel
provides for a high quality printer having a reduced cost since a printer
can operate effectively without the need for high cost encoders in the
paper advance direction.
In recapitulation, there has been described a liquid ink printer having
multiple grayscale printing. Multiple level grayscale printing is enabled
by providing a printer having a scanning carriage which can be operated at
a variety of speeds such that for each speed of the carriage a different
plurality of drops per pixel is selected to thereby produce multiple
levels of grayscale. While the present invention has been described in
conjunction with a specific embodiment thereof, it is evident that many
alternatives, modifications, and variations will be apparent to those
skilled in the art. For instance, the present invention is not limited to
thermal ink jet printing but includes all types of printing where liquid
drops are deposited on a recording medium or an intermediate including but
not limited to acoustic ink jet printing as well as phase change wax based
printing. In addition, the present invention is not limited to color
printing but includes monochrome printing as well. Accordingly, the
present invention is intended to embrace all such alternatives,
modifications, and variations that fall within the spirit and broad scope
of the appended claims.
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